Wrong Match History

When I was playing League with a friend, he checked out my match history and told me that I got like 35 kills. As much as I'd like to say I did, the most kills I could get in a game is like 2. Ever since he pointed that out, there were tons more mixups, and in some of them, it said I left the game, which isn't true, I've only left 1 game (or two) ever since I started playing league. More matches on the day I didn't play started to appear in my history and I don't know why.